1. Field of the Invention
This invention relates to a current control device and more particularly to such a control device for a stepping motor.
2. Description of the Prior Art From the U.S. Pat. No. 4,129,810, "Switching Motor Control System", filed May 3, 1976 by Robert P. Harshberger, Jr., is known a current control device for a DC motor. This known device comprises a motor coil, a positive first current source and a negative second current source to which the motor coil can be alternately connected by means of switching means. These switching means are controlled by control means which comprise comparing means for comparing a measuring signal with a reference signal. The measuring signal is proportional to the actual coil current; the reference signal is proportional to a (desired) motor or coil parameter (at a certain moment), e.g. the motor velocity or the coil current.
If there is a difference between the measuring signal and the reference signal, the amplitude of this difference will be converted into a binary control (error) signal. This binary control signal is a bistable digital signal which indicates the (size of the) difference between the measuring signal and the reference signal by relative time durations of two different stable signal levels of the digital control signal. The first signal level causes the switching means to connect the motor coil to the first current source; the second signal level causes the switching means to connect the motor coil to the second current source respectively. The digital control signal continues to switch the switching means with an average period corresponding to a (fixed) clock (and sample) frequency. In other words, the amplitude of said difference between the measuring signal and the reference signal is converted into a binary, pulse width modulated signal of a certain (fixed) frequency.
A serious disadvantage of the known current control device is the required relatively high frequency of the binary control signal. For this reason the said switching means require a high energy dissipation capacity. The known control device cannot be applied to control a stepping motor, especially a stepping motor which is able to make very little steps ("microsteps") with a high velocity. The reason is that for achieving microsteps with a high velocity the reference signal has to change very quickly. The required frequency of the binary control signal for this would be thus high that, due to the self-inductance of the coils, the motor torque would be very limited. In other words, the stepping size, the stepping velocity and the motor torque of a (micro)stepping motor controlled by the known control device would be intolerably limited. In addition, if the known control device would control a microstepping motor, the minimum stepping size (given a certain velocity) would always depend on the (fixed) sample frequency of the binary control signal. Thus it would never be possible to achieve steps which are smaller than a certain limited size, due to the fixed nature of the binary control signal frequency.
Concluding, applying the known control device to a (micro)stepping motor suffers from the main disadvantage that the frequency of the binary control signal is fixed, independent of variations of the reference signal: sometimes the frequency is higher than required, causing too much energy dissipation in the switching means and causing limitations to the velocity, stepping size and torque of the motor due to self-inductance, sometimes the frequency is lower than required, causing a limitation of the motor velocity or stepping size.